In different types of laser machining of workpieces, in addition to a laser machining beam a process gas jet is directed onto the machining point. In laser cutting, for instance, oxygen or inert gases, for example nitrogen or argon, can be considered as process gases. Molten material that is formed in the course of the cutting process is intended to be blown away from the kerf by the process or cutting gases. During infeed of oxygen as the process gas, the cutting process is additionally assisted by oxidation processes at the machining point. In these processes, an appropriate position of the laser machining beam relative to the process gas jet is required. In the case of laser cutting, optimum machining results are achieved when the laser beam axis of the laser machining beam directed onto the workpiece and the gas jet axis of the cutting gas jet directed onto the workpiece coincide with one another. For a wide variety of reasons, for example due to contamination of the laser nozzle of the laser machining head, a desired relative positioning of the laser beam axis and the gas jet axis, once set, is not sustained for an unlimited time during operation of a laser machine tool. Thus, to achieve optimum machining results, the relative positions of the laser beam axis and the gas jet axis should be monitored and corrected upon undesirable variation from the desired position.
Methods of the generic kind provided for that purpose and a laser machine tool of the generic kind implementing these methods are known from DE 43 36 136 A1. In this prior art, the machining point of a laser machining beam is detected by means of a camera through the nozzle orifice of a laser nozzle on a laser machining head. The offset, if applicable, of the machining point, and thus of the laser beam axis of the laser machining beam, with respect to the centre of the nozzle orifice on the laser nozzle defining the position of the gas jet axis of a process gas jet is determined by signal evaluation. The determined offset of the laser beam axis and gas jet axis ultimately forms the basis for a correction of the relative position of the two axes.
A further method of the generic kind for determining the position of a laser beam axis of a laser cutting beam of a laser cutting machine relative to that of a gas jet axis of a cutting gas jet, is known from industrial practice under the name “tape shot method.” In this case, first of all the nozzle orifice of a laser nozzle on a laser cutting head is taped over with an adhesive tape. Then a laser cutting beam is directed briefly through the nozzle orifice. The position of the centre of the hole produced by the laser cutting beam in the adhesive tape relative to the middle of the laser nozzle is finally established as the relative position of laser beam axis and gas jet axis. On variation from the desired relative position, the position of the laser cutting beam or the cutting gas jet as the case may be is corrected to eliminate the positional variation.